Comparative Analyses of Deletion Mutations and the Pathological Effects of Nucleopolyhedroviruses Isolated from Saturniid Wild Silkworms

摘要

Nucleopolyhedroviruses (NPVs) of saturniid wild silkworms are important in both insect pathology and biotechnology, because they cause severe damage to silk production in Asian countries and are also utilized as expression vectors for the production of valuable proteins in wild silkworms with larger body sizes than the domesticated silkworm, Bombyx mori . In this study, partial genomic DNA sequences of the Antheraea yamamai NPV (AnyaNPV) were determined and compared with the corresponding sequences of the A. pernyi NPV (AnpeNPV) and Samia ricini NPV (SariNPV). Their sequences were highly homologous, suggesting that they might be variants derived from a common ancestral NPV. They also shared large deletions in the coding region of the ecdysteroid UDP-glucosyltransferase gene ( egt ), although a clone of SariNPV possessed a full-length egt . The 4th instar larvae of S. ricini infected with egt -deleted NPVs died earlier than those infected with a cloned SariNPV carrying the full-length egt , suggesting that the egt deletions reduce production of polyhedral occlusion bodies (POBs) in the infected insects. In addition, SariNPV possessed large deletions in the contiguous cathepsin and chitinase gene region ( v-cath~chiA ), and its infection to S. ricini larvae did not cause obvious liquefaction of the larval bodies as well as burst release of POBs, suggesting that the v-cath~chiA deletions reduce horizontal transmission in the insect population. These results seem to indicate that the deletions detected in the egt and/or v-cath~chiA regions of the saturniid NPVs may not be significantly disadvantageous and have some good reasons, at least, in their infection cycles under the rearing conditions of their hosts.